Differential device

ABSTRACT

A differential device includes pinion; pinion shaft; paired side gears connected to paired output shafts and having at outer peripheral portions gear portions meshing with the pinion; and oil introduction passage introducing lubricant oil to mutually-facing surfaces of the side gears. An oil reserving portion facing space adjacent to end surface of the pinion on radially inner side and capable of catching and holding lubricant oil splashed to the space is formed in portion where the pinion and the pinion shaft mutually face, so as to communicate with fitting portion between the pinion and the pinion shaft where they are relatively rotationally slidable. A step portion is formed in at least one of the mutually-facing surfaces, separates part of oil from lubricant flow flowing radially outward along the one surface by centrifugal force, and guides the oil into the space.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a differential device whichdistributively transmits rotational force of a differential case to apair of output shafts.

Description of the Related Art

Japanese Patent Application KOKAI Publication No. 2014-190374, forexample, has made known a differential device which distributivelytransmits rotational force of a differential case to a pair of outputshafts, comprising: a pinion (differential gear) placed in thedifferential case; a pinion shaft (differential gear support portion)supported in the differential case and penetrating through and rotatablysupporting the pinion; and a pair of side gears (output gears) includingat respective outer peripheral portions thereof gear portions which areplaced in mesh with the pinion, the side gears facing each other withthe pinion shaft interposed therebetween and being connectedrespectively to the pair of output shafts, wherein, conventionally, forexample, an oil reserving portion is provided in a portion where thepinion and the pinion shaft face each other such that the oil reservingportion faces a space adjacent to an end surface of the pinion that islocated on a radially inner side of the side gears, and lubricant oil isintroduced to mutually-facing surfaces of the pair of side gears via alubricant oil supply passage provided between inner peripheral shaftportions of the side gears and the output shafts and then part of theintroduced lubricant oil can be supplied to the oil reserving portion.

In the conventional device, however, the lubricant oil introduced intoan interstice between the mutually-facing surfaces of the pair of sidegears tends to flow outward in the radial direction along themutually-facing surfaces due to centrifugal force. As a result, most ofthe introduced lubricant oil flows directly toward the gear portions inouter peripheries of the side gears continuous to the mutually-facingsurfaces. For this reason, an effect of lubricating meshing portions ofthe side gears and the pinion can be sufficiently obtained. On the otherhand, however, an amount of lubricant oil supplied to the oil reservingportion is relatively small. This arises a problem that an effect oflubricating a fitting portion, that is, a rotation sliding portion,between the pinion and the pinion shaft cannot be sufficiently obtained.

In addition, the problem of the insufficient lubrication of the rotationsliding portion between the pinion and the pinion shaft may be moreconspicuous particularly under severe driving conditions with high-speedrotation of the pinion as in a case, for example, where a width of thedifferential case is reduced in an axial direction of the output shaftsby making a diameter of each side gear sufficiently larger than adiameter of the pinion so that the number of teeth of the side gear canbe set sufficiently larger than the number of teeth of the pinion.

SUMMARY OF THE INVENTION

The present invention has been made with the foregoing situation takeninto consideration. An object of the present invention is to provide adifferential device capable of solving the above-mentioned problem.

In order to achieve the object, a differential device according to thepresent invention, which distributively transmits rotational force of adifferential case to a pair of output shafts, comprising: a pinionplaced in the differential case; a pinion shaft supported in thedifferential case and penetrating through and rotatably supporting thepinion; a pair of side gears including at respective outer peripheralportions thereof gear portions which are placed in mesh with the pinion,the side gears facing each other with the pinion shaft interposedtherebetween and being connected respectively to the pair of outputshafts; and an oil introduction passage introducing lubricant oil tomutually-facing surfaces of the pair of side gears, wherein an oilreserving portion is formed in a portion where the pinion and the pinionshaft face each other such that the oil reserving portion communicateswith a fitting portion between the pinion and the pinion shaft, wherethe pinion and the pinion shaft are rotationally slidable relative toeach other, the oil reserving portion facing a space adjacent to an endsurface of the pinion that is located on a radially inner side, the oilreserving portion being capable of catching and holding the lubricantoil splashed to the space, and a step portion is formed in at least oneof the mutually-facing surfaces of the pair of side gears, the stepportion separating part of the lubricant oil from a lubricant oil flowthat flows due to centrifugal force in a radially outward directionalong the one of the mutually-facing surfaces and guiding the part ofthe lubricant oil into the space. (This is a first characteristic of thepresent invention.)

Further, in order to achieve the object, a differential device accordingto the present invention, which distributively transmits rotationalforce of a differential case to a pair of output shafts, comprising: apinion placed in the differential case; a pinion shaft supported in thedifferential case and penetrating through and rotatably supporting thepinion; a pair of side gears including at respective outer peripheralportions thereof gear portions which are placed in mesh with the pinion,the side gears facing each other with the pinion shaft interposedtherebetween and being connected respectively to the pair of outputshafts; and an oil introduction passage introducing lubricant oil tomutually-facing surfaces of the pair of side gears; a space interposedbetween recesses formed in the mutually-facing surfaces of the pair ofside gears and an end surface of the pinion that is located on aradially inner side of the side gears; and an oil reserving portionformed in a portion where the pinion and the pinion shaft face eachother such that the oil reserving portion faces the space, the oilreserving portion communicating with a fitting portion between thepinion and the pinion shaft where the pinion and the pinion shaft arerotationally slidable relative to each other, wherein a step portion isformed in at least one of the mutually-facing surfaces of the pair ofside gears, the step portion being located in an inner peripheral end ofthe space in the radial direction and forming an opening edge of thecorresponding recess. (This is a second characteristic of the presentinvention.)

According to the first and second characteristics, the step portionformed in at least one of the mutually-facing surfaces of the pair ofside gears is capable of: effectively separating the part of thelubricant oil from the lubricant oil flow that flows due to thecentrifugal force in the radially outward direction of the side gearsalong the mutually-facing surfaces toward the gear portions of the sidegears; and guiding and splashing the separated lubricant oil into thespace adjacent to the end surface of the pinion that is located on theradially inner side of the side gears. Thus, the splashed lubricant oilcan be efficiently caught and held in the oil reserving portion which isformed in the portion where the pinion and the pinion shaft face eachother such that the oil reserving portion faces the space. Thereby, thelubricant oil can be sufficiently supplied to the fitting portion, thatis, the rotation sliding portion, between the pinion and the pinionshaft. Accordingly, even in a case of severe driving conditions such ashigh-speed rotation of the pinion, and the like, not only can themeshing portions of the pinion and the side gears be lubricated, butalso the rotation sliding portion between the pinion and the pinionshaft can be sufficiently lubricated. Thus, seizure in the meshingportions and the rotation sliding portion can be effectively preventedusing the simple structure.

Further, in order to achieve the object, a differential device accordingto the present invention, which distributively transmits rotationalforce of a differential case to a pair of output shafts, comprising: adifferential gear placed in the differential case; a differential gearsupport portion supported in the differential case and penetratingthrough and rotatably supporting the differential gear; a pair of outputgears including at respective outer peripheral portions thereof gearportions which are placed in mesh with the differential gear, the outputgears facing each other with the differential gear support portioninterposed therebetween and being connected respectively to the pair ofoutput shafts; and an oil introduction passage introducing lubricant oilto mutually-facing surfaces of the pair of output gears; a spaceinterposed between recesses formed in the mutually-facing surfaces ofthe pair of output gears and an end surface of the differential gearthat is located on a radially inner side of the output gears; and an oilreserving portion formed in a portion where the differential gear andthe differential gear support portion face each other such that the oilreserving portion faces the space, the oil reserving portioncommunicating with a fitting portion between the differential gear andthe differential gear support portion, where the differential gear andthe differential gear support portion are rotationally slidable relativeto each other, wherein a step portion is formed in at least one of themutually-facing surfaces of the pair of output gears, the step portionbeing located in an inner peripheral end of the space in the radialdirection and forming an opening edge of the corresponding recess,wherein

${d\; 2\text{/}{PCD}} \leqq {3.36 \cdot \left( \frac{1}{z\; 1} \right)^{\frac{2}{3}} \cdot {\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}$is satisfied, and

Z1/Z2>2 is satisfied, where Z1, Z2, d2 and PCD denote the number ofteeth of each of the output gears, the number of teeth of thedifferential gear, a diameter of the differential gear support portionand a pitch cone distance, respectively. (This is a third characteristicof the present invention.)

According to the third characteristic, the step portion formed in atleast one of the mutually-facing surfaces of the pair of output gears iscapable of: effectively separating the part of the lubricant oil fromthe lubricant oil flow that flows due to the centrifugal force in theradially outward direction of the output gears along the mutually-facingsurfaces toward the gear portions of the output gears; and guiding andsplashing the separated lubricant oil into the space adjacent to the endsurface of the differential gear on the inner side in the radialdirection of the output gears. Thus, the splashed lubricant oil can beefficiently caught and held in the oil reserving portion formed in theportion where the differential gear and the differential gear supportportion face each other such that the oil reserving portion faces thespace. Thereby, not only can the meshing portions of the differentialgear and the output gears be lubricated, but also the rotation slidingportion between the differential gear and the differential gear supportportion can be sufficiently lubricated. Thus, seizure in the meshingportions and the rotation sliding portions can be effectively preventedusing the simple structure. Furthermore, according to the thirdcharacteristic, the differential device as a whole can be sufficientlyreduced in width in the axial direction of the output shafts whilesecuring the strength (for example, the static torsion load strength)and the maximum amount of torque transmission at approximately the samelevels as the conventional differential device. Accordingly, thedifferential device can be easily incorporated in a transmission system,which is under many layout restrictions around the differential device,with great freedom and no specific difficulties, and is thereforeadvantageous in reducing the size of the transmission system.

In the differential device according to the present invention,preferably, the side gears include: shaft portions connectedrespectively to the pair of output shafts; the gear portions separatingoutward from the shaft portions in the radial direction; andintermediate wall portions each having a flat shape and extendingoutward from inner end portions of the shaft portions in the radialdirection. (This is a fourth characteristic of the present invention.)

According to the fourth characteristic, the side gears include: theshaft portions connected respectively to the pair of output shafts; thegear portions separating outward from the shaft portions in the radialdirection; and the intermediate wall portions each having a flat shapeand extending outward from the inner end portions of the shaft portionsin the radial direction. For this reason, the diameter of each side gearcan be made larger than the diameter of the pinion as much as possible,so that the number of teeth of the side gear can be set sufficientlylarger than the number of teeth of the pinion. This makes it possible toreduce load burden on the pinion shaft, and thus to decrease the size ofthe pinion shaft, and accordingly decrease a width of the differentialcase in the axial direction of the output shafts. In addition, theabove-described increase in the diameter of each side gear causes largercentrifugal force to be applied to the lubricant oil flow that flows inthe radially outward direction of the side gears along themutually-facing surfaces (that is, the intermediate wall portionsrelatively wide in the radial direction). This enhances the effect ofseparating the part of the lubricant oil from the lubricant oil flow andsplashing the part of the lubricant oil using the step portion, andaccordingly makes it possible for the splashed lubricant oil to be moreeffectively caught in the oil reserving portion.

In the differential device according to the present invention,preferably, Z1/Z2≧4 is satisfied. (This is a fifth characteristic of thepresent invention.)

In the differential device according to the present invention,preferably, Z1/Z2≧5.8 is satisfied. (This is a sixth characteristic ofthe present invention.)

According to the fifth and sixth characteristics, the differentialdevice can be more sufficiently reduced in width in the axial directionof the output shafts while securing the strength (for example, thestatic torsion load strength) and the maximum amount of torquetransmission at approximately the same levels as the conventionaldifferential device.

In the differential device according to the present invention,preferably, the step portion is formed such that an imaginary planepasses through any one of an inner space and an opening edge of the oilreserving portion, the imaginary plane passing through a top surface ofthe step portion and being orthogonal to a rotation axis of thedifferential case. (This is a seventh characteristic of the presentinvention.)

According to the seventh characteristic, the step portion is formed suchthat the imaginary plane passing through the top surface of the stepportion and being orthogonal to the rotation axis of the differentialcase passes through the inner space or the opening edge of the oilreserving portion. Thus, using the step portion, the lubricant oil isseparated from the lubricant oil flow that flows due to the centrifugalforce in the radially outward direction of the side gears (output gears)along the mutually-facing surfaces, and is guided and splashed to thespace. Then, the splashed lubricant oil can be effectively caught andsufficiently held in the oil reserving portion. Therefore, it ispossible to efficiently supply the lubricant oil to the rotation slidingportion between the pinion (differential gear) and the pinion shaft(differential gear support portion).

The above and other objects, characteristics and advantages of thepresent invention will be clear from detailed descriptions of thepreferred embodiment which will be provided below while referring to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a main part in a differentialdevice and a speed reduction gear mechanism according to an embodimentof the present invention (a sectional view taken along a 1A-1A line inFIG. 2).

FIG. 2 is a partially cutaway side view on one side in an axialdirection of the differential device (a sectional view taken along a2A-2A line in FIG. 1).

FIG. 3 is a side view of a main part on the other side in the axialdirection of the differential device (a sectional view taken along a3A-3A line in FIG. 1).

FIG. 4 is a sectional view taken along a 4A-4A line in FIG. 1 and showsonly one cover portion C with solid lines.

FIG. 5 is a sectional view taken along a 5A-5A line in FIG. 1 and showsonly the other cover portion C′ and a differential case with solidlines.

FIG. 6A is an enlarged view of a section indicated with an arrow 6A inFIG. 1 and FIG. 6B is a sectional view taken along a BA-BA line in FIG.6A.

FIG. 7 is a longitudinal sectional view showing an example of aconventional differential device.

FIG. 8 is a graph showing a relationship of gear strength change rateswith a number-of-teeth ratio where the number of teeth of the pinion isset at 10.

FIG. 9 is a graph showing a relationship of the gear strength changerates with a pitch cone distance change rate.

FIG. 10 is a graph showing a relationship of the pitch cone distancechange rate with the number-of-teeth ratio for keeping 100% of the gearstrength where the number of teeth of the pinion is set at 10.

FIG. 11 is a graph showing a relationship between a shaft diameter/pitchcone distance ratio and the number-of-teeth ratio where the number ofteeth of the pinion is set at 10.

FIG. 12 is a graph showing a relationship between the shaftdiameter/pitch cone distance ratio and the number-of-teeth ratio wherethe number of teeth of the pinion is set at 6.

FIG. 13 is a graph showing a relationship between the shaftdiameter/pitch cone distance ratio and the number-of-teeth ratio wherethe number of teeth of the pinion is set at 12.

FIG. 14 is a graph showing a relationship between the shaftdiameter/pitch cone distance ratio and the number-of-teeth ratio wherethe number of teeth of the pinion is set at 20.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below based onthe attached drawings.

First of all, in FIG. 1, a differential device D is connected to anengine (not illustrated) as a power source mounted on an automobile viaa speed reduction gear mechanism RG. The differential device D drivesleft and right axles while allowing differential rotation between theleft and right axles by distributively transmitting to a pair of leftand right output shafts J rotational force which is transmitted from theengine to a differential case DC via the speed reduction gear mechanismRG, the pair of left and right output shafts J being continuousrespectively to the pair of left and right axles. The differentialdevice D is housed together with the speed reduction gear mechanism RGin, for example, a transmission case M placed beside the engine in afront portion of a vehicle body, in a way that the differential device Dis adjacent to the speed reduction gear mechanism RG. Incidentally, apower connection-disconnection mechanism and a forward-rearward travelswitching mechanism (both not illustrated) which have been well-knownare installed between the engine and the speed reduction gear mechanismRG. In addition, a rotation axis (rotation center, center axis) L of thedifferential case DC coincides with a center axis of the output shaftsJ.

In the illustrated example, the speed reduction gear mechanism RG isformed from a planetary gear mechanism including: a sun gear 50 whichrotates in operative connection with a crankshaft of the engine; a ringgear 51 which concentrically surrounds the sun gear 50 and is fixed toan inner wall of the transmission case M; a plurality of planetary gears52 which are installed between the sun gear 50 and the ring gear 51 andmesh with them; and a carrier 53 which rotatably and pivotally supportsthe planetary gears 52. Incidentally, a speed reduction gear mechanismformed from a gear train including multiple spur gears may be usedinstead of such a planetary gear mechanism.

The carrier 53 is rotatably supported by the transmission case M via abearing (not illustrated). Furthermore, the carrier 53 is joined to oneend portion of the differential case DC of the differential device D soas to rotate integrally with the differential case DC. An other endportion of the differential case DC is rotatably supported in thetransmission case M via a bearing 2. A combination body of thedifferential case DC and the carrier 53 which integrally rotate togetheris rotatably and stably supported in the transmission case M via themultiple bearings.

In addition, a through-hole Ma to be inserted with each of the outputshafts J is formed in the transmission case M. A seal member 3 having anannular shape and sealing an interstice between an inner periphery ofthe through-hole Ma and an outer periphery of each output shaft J isinstalled therebetween. Furthermore, an oil pan (not illustrated) whichfaces an inner space 1 of the transmission case M and holds apredetermined amount of lubricant oil is provided in a bottom portion ofthe transmission case M. In the inner space 1 of the transmission caseM, the lubricant oil held in the oil pan is scraped up and splashed byrotation of movable elements of the speed reduction gear mechanism RG,the differential case DC and the like toward vicinities of rotationalparts. This makes it possible to lubricate the mechanical moving partsexisting inside and outside the differential case DC. Incidentally, thelubricant oil may be sucked in by an oil pump (not illustrated) to beforcibly splashed or sprayed toward specific parts in the inner space 1of the transmission case M, for example toward the speed reduction gearmechanism RG and the differential case DC, or toward an inner wall ofthe transmission case M in peripheries of the speed reduction gearmechanism RG and the differential case DC.

Meanwhile, as being clear from FIG. 1, a ceiling wall Mt of thetransmission case M includes an inclined portion descending toward aportion right above the differential case DC. Part of the lubricant oilsplashed inside the transmission case M as described above adheres alsoto the ceiling wall Mt of the transmission case M, subsequently flowstoward a lower portion of the ceiling wall Mt along an inclined innersurface Mtf of the ceiling wall Mt, and thereafter drips down from aspecific part of the ceiling wall Mt, for example from a terminal partof the inclined inner surface Mtf (that is, a boundary part between theinclined inner surface Mtf and a horizontal surface of the ceiling wallMt) toward the differential case DC right under the specific part of theceiling wall Mt. This makes it possible to take part of the drippinglubricant oil into oil intake holes H1, H2 described later which areopened in an outer peripheral surface of the differential case DC.Incidentally, even if the ceiling wall Mt of the transmission case Mdoes not include the above-described inclined portion, the adheringlubricant oil randomly drips down from parts of an inner surface of theceiling wall Mt due to its own weight because the large amount oflubricant oil is splashed and adheres onto the ceiling wall Mt.Accordingly, part of the lubricant oil can be taken into the oil intakeholes H1, H2.

Referring to FIGS. 2 to 6 together, the differential device D includes:the differential case DC; a plurality of pinions (differential gears) Phoused in the differential case DC; a pinion shaft (differential gearsupport portion) PS housed in the differential case DC and rotatablysupporting the pinions P; and a pair of left and right side gears(output gears) S housed in the differential case DC, meshing with thepinions P respectively from both the left and right sides and connectedrespectively to the pair of left and right output shafts J. Furthermore,the differential case DC includes: a case main body 4 having a shortcylindrical shape and supporting the pinion shaft PS so as to be able torotate with the pinion shaft PS; and a pair of left and right coverportions C, C′ respectively covering outer sides of the both side gearsS and rotating integrally with the case main body 4. The case main body4 forms an outer peripheral wall of the differential case DC.

The pinion shaft PS is arranged to cross the rotation axis L of thedifferential case DC inside the differential case DC. Both end portionsof the pinion shaft PS are removably inserted into a pair of supportthrough-holes 4 a which are provided to the case main body 4 and arearranged on one diameter line of the case main body 4. Furthermore, thepinion shaft PS is fixed to the case main body 4 using a retaining pin 5which penetrates through the one end portion of the pinion shaft PS andis inserted into the case main body 4. In a state where the pinion shaftPS is fixed to the case main body 4, both outer end surfaces PSf of thepinion shaft PS face the inner space 1 of the transmission case Mthrough openings DCo in the outer peripheral surface of the differentialcase DC (that is, openings of outer ends of the support through-holes 4a).

The embodiment shows the differential device D which includes twopinions P, and whose pinion shaft PS is formed in a linear rod shapeextending along one diameter line of the case main body 4 with the twopinions P respectively supported by both end portions of the pinionshaft PS. Instead, the differential device D may include three or morepinions P. In this case, the pinion shaft PS is formed in a shape ofcrossing rods such that rods extend radially from a rotation axis L ofthe differential case DC in three or more directions corresponding tothe three or more pinions P (for example, in a shape of a cross when thedifferential device D includes four pinions P), and tip end portions ofthe pinion shaft PS support the pinions P, respectively. In addition,the case main body 4 is formed from two dividing elements, and thepinion shafts PS is interposed between the dividing elements.

Moreover, each pinion P may be directly fitted to the pinion shaft PS,as in the illustrated example. Otherwise, the pinion P may be fitted tothe pinion shaft PS via bearing means (not illustrated) such as abearing bush and the like. In the former case, a fitting portion betweenthe pinion shaft PS and the pinion P forms a rotation sliding portion rsbetween the pinion shaft PS and the pinion P. In the latter case, theabove-mentioned bearing means forms the rotation sliding portion rs.Incidentally, as shown in the illustrated example, the pinion shaft PSmay be formed in a shape of a shaft whose diameter is substantiallyequal throughout its whole length, or formed in a shape of a steppedshaft.

Meanwhile, in the embodiment, the pinions P and the side gears S areeach formed as a bevel gear. In addition, each pinion P as a whole andeach side gear S as a whole, including their tooth portions, are formedby plastic working such as forging and the like. For these reasons,their tooth portions with an arbitrary gear ratio can be preciselyformed without restriction in machining work in the case where the toothportions of the pinions P and the side gears S are formed by cuttingwork. Incidentally, other types of gears may be used instead of thebevel gear. For example, a face gear may be used for the side gears S,while a spur gear or a helical gear may be used for the pinions P.

In addition, the pair of side gears S each include: a shaft portion Sjto which an inner end portion of the corresponding one of the pair ofoutput shafts J is spline-fitted as at 6 and being formed in acylindrical shape; a tooth portion Sg situated at a position separatedoutward from the shaft portion Sj in a radial direction of thedifferential case DC, being in mesh with the corresponding pinion P andbeing formed in an annular shape; and an intermediate wall portion Swformed in a flat ring plate shape orthogonal to the axis L of thecorresponding output shaft J and integrally connecting the shaft portionSj and the tooth portion Sg. Incidentally, in the illustrated example,the shaft portions Sj of the side gears S are directly and rotatablyfitted in boss portions Cb of the cover portions C, C′, respectively,but may be rotatably fitted in the boss portions Cb of the coverportions C, C′ via bearings, respectively.

In the intermediate wall portion Sw of at least one (in the embodiment,each of the two) of the left and right side gears S, penetrating oilpassages 15 are formed in the intermediate wall portion Sw so as tocross the intermediate wall portion Sw, both ends of each penetratingoil passage 15 being respectively opened in inner and outer surfaces ofthe intermediate wall portion Sw.

In addition, the intermediate wall portion Sw of the side gear S isformed with its width t1 in the radial direction larger than a maximumdiameter d1 of the pinion P, and with its maximum thickness t2 in anaxial direction of the output shaft J smaller than an effective diameterd2, that is, an outer diameter, of the pinion shaft PS (see FIG. 1).Thereby, as described later, a diameter of the side gear S can be madelarge enough to set the number Z1 of teeth of the side gear Ssufficiently larger than the number Z2 of teeth of the pinions P, andthe side gear S can be sufficiently thinned in the axial direction ofthe output shaft J.

One of the pair of left and right cover portions C, C′ in thedifferential case DC, for example, the cover portion C which is on anopposite side to the speed reduction gear mechanism RG is formedseparately from the case main body 4 and detachably joined to the casemain body 4 using bolts B. Various joining means other than the screwingmeans, for example, welding means and caulking means may be also used tojoin the cover portion C to the case main body 4. Moreover, in theillustrated example, the other cover portion C′ is integrally formed inthe case main body 4 and joined to the carrier 53 of the speed reductiongear mechanism RG. However, like the cover portion C, the other coverportion C′ may be formed separately from the case main body 4 and joinedto the case main body 4 using the bolts B or other joining means.

Besides, each of the cover portions C, C′ includes: a boss portion Cbwhich concentrically surrounds the shaft portion Sj of the side gear S,in which the shaft portion Sj is rotatably fitted and supported andbeing formed in a cylindrical shape; and a side wall portion Cs havingan outer side surface which is a flat surface orthogonal to the rotationaxis L of the differential case DC, the side wall portion Cs beingintegrally connected to an inner end in an axial direction of the bossportion Cb and being formed in a plate shape. The side wall portions Csof the cover portions C, C′ are arranged within a width of the case mainbody 4 in an axial direction of the output shafts J. This inhibits theside wall portions Cs of the cover portions C, C′ from protrudingoutward from end surfaces of the case main body 4 in the axialdirection, and therefore it is advantageous for a reduction in the widthof the differential device D in the axial direction of the output shaftsJ.

Besides, back surfaces of at least one of the intermediate wall portionsSw and the tooth portions Sg (in the illustrated example, theintermediate wall portions Sw) of the side gears S are rotatablysupported on inner side surfaces of the side wall portions Cs of thecover portions C, C′ via washers W. Incidentally, the back surfaces ofthe side gears S may be directly and rotatably supported on the innerside surfaces of the side wall portions Cs by omitting such washers W.

In addition, each side gear S of the embodiment includes theintermediate wall portion Sw having a flat ring plate shape andintegrally connecting between the shaft portion Sj on an innerperipheral side of the side gear S and the tooth portion Sg on an outerperipheral side of the side gear S, the tooth portion Sg being separatedoutward from the shaft portion Sj in a radial direction of the side gearS. The width t1 in the radial direction of the intermediate wall portionSw is larger than the maximum diameter d1 of each pinion P. For thesereasons, the diameter of each side gear S can be made sufficientlylarger than the diameter of the pinion P, so that the number Z1 of teethof the side gear S can be made sufficiently larger than the number Z2 ofteeth of the pinion P. This makes it possible to reduce load burden onthe pinion shaft PS in torque transmission from the pinions P to theside gears S, and thus to decrease the effective diameter d2 of thepinion shaft PS, and accordingly decrease a width (diameter) of eachpinion P in the axial direction of the output shafts J.

In addition, since the load burden to the pinion shaft PS is reduced asdescribe above, since reaction force applied to each side gear Sdecreases and since the back surface of the intermediate wall portion Swof the side gear S is supported by the corresponding side wall portionCs of each of the cover portions C, C′, it is easy to secure therigidity strength needed for the side gear S even though theintermediate wall portion Sw is thinned. That is, it is possible tosufficiently thin the intermediate wall portion Sw of the side gear Swhile securing the support rigidity with respect to the side gear S.Moreover, in the embodiment, since the maximum thickness t2 of theintermediate wall portion Sw of the side gear S is formed much smallerthan the effective diameter d2 of the pinion shaft PS whose diameter canbe made smaller, the further thinning of the intermediate wall portionSw of the side gear S can be achieved. Besides, since the side wallportion Cs of each of the cover portions C, C′ is formed in a plateshape such that the outer side surface thereof is the flat surfaceorthogonal to the rotation axis L of the differential case DC, thethinning of the side wall portion Cs itself can be achieved.

As a result of these, the width of the differential device D as a wholecan be sufficiently decreased in the axial direction of the outputshafts J while securing as approximately the same strength (for example,static torsion load strength) and as approximately the same amount ofmaximum torque transmission compared with the conventional differentialdevice. This makes it possible to easily incorporate the differentialdevice D, with great freedom and without trouble, even when atransmission system imposes many restrictions on the layout of thevicinity of the differential device D, and is extremely advantageous inreducing the size of the transmission system.

Meanwhile, the side wall portion Cs of the one cover portion C has astructure having oil retaining portions 7 covering parts of a backsurface of the side gear S in first predetermined areas including areaswhich overlap the pinions P as seen in a side view from outside in theaxial direction of the output shaft J (i.e., as seen in FIG. 2), havinglightening portions 8 exposing parts of the back surface of the sidegear S to the outside of the differential case DC in secondpredetermined areas which do not overlap the pinions P as seen in theside view and having connecting arm portions 9 being separated from theoil retaining portions 7 in the peripheral direction of the case mainbody 4, extending in the radial direction of the case main body 4 andconnecting between the boss portion Cb and the case main body 4. Inother words, the side wall portion Cs basically having a disk shape inthe cover portion C has a structural form in which: the plurality oflightening portions 8 each having a cutout shape are formed in the sidewall portion Cs at intervals in the peripheral direction; and thereby,one oil retaining portion 7 and one connecting arm portion 9 are formedrespectively on opposite sides of the lightening portion 8 in theperipheral direction.

The structural form of the side wall portion Cs of the cover portion C,particularly the oil retaining portions 7, makes it possible for thelubricant oil to easily stay in spaces covered by the oil retainingportions 7 and the case main body 4 and to be easily retained around thepinions P and the vicinities of the pinions P, the lubricant oil tendingto move outward in the radial direction due to the centrifugal forceproduced by the rotation of the differential case DC.

Furthermore, as shown in FIG. 3, in this embodiment, the lighteningportions 8 are formed in the side wall portion Cs of the other coverportion C′, like in the one cover portion C. In the side wall portion Csof the other cover portion C′, however, the oil retaining portions 7 andthe connecting arm portions 9 are integrally formed in the case mainbody 4. Incidentally, the side wall portion Cs of one of the coverportions C, C′ may be formed in a disk shape having no lighteningportions (accordingly covering the entirety of the back surfaces of theintermediate wall portion Sw and the tooth portion Sg of thecorresponding side gear S).

Meanwhile, as clearly shown in FIGS. 6A and 6B, inside the differentialcase DC, an oil reserving portion 61 is formed a portion where thepinion P and the pinion shaft PS face each other such that the oilreserving portion 61 directly communicates with the fitting portion(that is, the rotation sliding portion rs) between the pinion P and thepinion shaft PS where they are rotationally slidable relative to eachother. The oil reserving portion 61 faces a space 60 adjacent to an endsurface Pfi of the pinion P on a radially inner side of the side gear Sand is capable of catching and holding the lubricant oil splashed in thespace 60. In the illustrated example, the oil reserving portion 61 isformed by performing a chamfering on an end edge of an inner peripheralsurface of the pinion P on the radially inner side of the side gear S,the chamfering being formed in an annular shape. The space 60 is formedso as to be interposed between a recess Swa formed in each ofmutually-facing surfaces of the pair of left and right side gears S (inthe illustrated example, the inner side surface of the intermediate wallportion Sw of each side gear S on the tooth portion Sg side) and the endsurface Pfi of the pinion P on the radially inner side of the side gearS. A step portion E having an edge shape is formed in each of themutually-facing surfaces of the pair of left and right side gears S. Thestep portion E is situated at an inner peripheral edge of the space 60in the radial direction of the side gear S and forms an opening edge ofthe recess Swa.

In other words, the step portion E having the edge shape and an annularshape is formed in each of the mutually-facing surfaces of the pair ofleft and right side gears S (in the illustrated example, the inner sidesurface of the intermediate wall portion Sw of each side gear S on thetooth portion Sg side), the step portion E being capable of guiding andsplashing part of the lubricant oil into the space 60 by separating thepart of the lubricant oil from a lubricant oil flow that flows due tocentrifugal force in the radially outward direction along the inner sidesurface of the intermediate wall portion Sw. A top surface of the stepportion E is flush with and connected to the inner side surface of theintermediate wall portion Sw which is inward of the step portion E inthe radial direction of the side gear S.

Meanwhile, each side gear S can be formed by forging or any otherforming method. In a case where, for example, the side gear S is formedby forging, there is likelihood that part of the side gear S between thetop surface of the step portion E and an outer peripheral surface (stepsurface) continuous to the step portion E is rounded due to shear droop.In this case, a sharp edge can be formed between the top surface and theouter peripheral surface (step surface) by machining the outerperipheral surface (step surface).

It should be noted that while the automobile is traveling in a forwarddirection and the differential case DC is being rotationally driven in anormal rotation direction R, the lubricant oil is efficiently suppliedto the vicinity of an intermediate portion in the radial direction ofthe inner side surface of the intermediate wall portion Sw via thepenetrating oil passages 15 in the side gear S, as described later. Forthis reason, the lubricant oil supplied to the vicinity of theintermediate portion in the radial direction flows due to thecentrifugal force in the radially outward direction along the inner sidesurface of the intermediate wall portion Sw, that is, toward the toothportion Sg. On the way to the tooth portion Sg, the lubricant oilreaches the step portion E.

Thereafter, the step portion E is capable of guiding and splashing partof the lubricant oil into the space 60 by effectively separating thepart of the lubricant oil from the lubricant oil flow that flows due tothe centrifugal force in the radially outward direction along the innerside surface of the intermediate wall portion Sw, using the edge portionof the step portion E. Thereby, the splashed lubricant oil can beefficiently caught and held in the oil reserving portion 61 facing thespace 60. For this reason, the lubricant oil is sufficiently supplied tothe rotation sliding portion rs between the pinion P and the pinionshaft PS via the oil reserving portion 61. Meanwhile, the remaining partof the lubricant oil flows toward the tooth portions Sg of the sidegears S along the step surface of the step portion E without beingsplashed from the edge portion of the step portion E. Accordingly,meshing portions of the tooth portions Sg and the pinion P can belubricated sufficiently. Thereby, even under severe driving conditionssuch as high-speed rotation of the pinion P due to a reduction in thediameter of the pinion P, and the like, both the meshing portions andthe rotation sliding portion rs between the pinion P and the pinionshaft PS are lubricated sufficiently.

Furthermore, the step portion E having the edge shape of the embodimentis formed in a way that an imaginary plane fe passing through the topsurface of the step portion E and orthogonal to the rotation axis L ofthe differential case DC passes through an inner space 61 s or anopening edge 61 e of the oil reserving portion 61. Thereby, thelubricant oil separated by the step portion E from the lubricant oilflow, guided and splashed into the space 60 can be effectively caught inthe oil reserving portion 61, and easily held in the oil reservingportion 61. For this reason, the lubricant oil can be more efficientlysupplied to the rotation sliding portion rs between the pinion P and thepinion shaft PS. What is more, as described above, the differentialdevice D of the embodiment employs the structure in which the diameterof the side gear S is made sufficiently larger than the diameter of thepinion P in order to reduce the width of the differential case DC in theaxial direction of the output shafts J. Accordingly, the increase in thediameter of the side gear S causes larger centrifugal force to beapplied to the lubricant oil flow that flows in the radially outwarddirection along the inner side surface of the intermediate wall portionSw. This enhances the effect of separating the part of the lubricant oilfrom the lubricant oil flow and splashing the part of the lubricant oilusing the step portion E, and accordingly makes it possible for thesplashed lubricant oil to be more effectively caught in the oilreserving portion 61.

Meanwhile, in the embodiment, as described above, the both outer endsurfaces PSf of the pinion shaft PS face the inner space 1 of thetransmission case M through the respective openings DCo in the outerperipheral surface of the differential case DC (that is, the openings ofthe outer ends of the support through-holes 4 a of the case main body4). In addition, as shown in FIGS. 6A and 6B, bottomed hollow portions Teach having one end opened and the other end closed are formedrespectively in the both end portions of the pinion shaft PS so as to berecessed from the both outer end surfaces PSf of the pinion shaft PS.Each bottomed hollow portion (hollow cylindrical portion) T is formed ina bottomed cylindrical hole shape which extends long in an axialdirection of the pinion shaft PS. The depth of the hole of the bottomedhollow portion T is set large enough for the hole to pass through therotation sliding portion rs between the pinion shaft PS and the pinion Pand to further extend inward of the differential case DC in the radialdirection. Thus, the bottomed hollow portion T employs a placement modein which at least an intermediate portion of the bottomed hollow portionT is concentrically surrounded by the rotation sliding portion rs.

A plurality of oil guiding holes G capable of guiding the lubricant oilheld in the bottomed hollow portion T to the rotation sliding portion rsdue to the centrifugal force are provided in a peripheral wall of thebottomed hollow portion T in the pinion shaft PS. Each of the oilguiding holes G is formed to cross the peripheral wall of the bottomedhollow portion T from an inner periphery to an outer periphery of theperipheral wall thereof and be inclined outward in the axial directionof the pinion shaft PS. The plurality of oil guiding holes G are formedto be arranged at intervals in a longitudinal direction of the bottomedhollow portion T. Furthermore, multiple groups each including thethus-arranged oil guiding holes G are arranged at intervals in aperipheral direction of the bottomed hollow portion T, that is, radiallyfrom a center axis of the bottomed hollow portion T. Moreover, anopening end Gi of each oil guiding hole G in the inner periphery of theperipheral wall of the bottomed hollow portion T is separated from abottom surface b of the bottomed hollow portion T in the longitudinaldirection of the bottomed hollow portion T. For this reason, a hollowpart Ta of the bottomed hollow portion T located between the bottomsurface b and the opening end Gi can function as an oil reservoir whichis capable of holding a necessary amount of lubricant oil.

Because of this specialized structure of the bottomed hollow portion Tin the pinion shaft PS, when the engine stops, the bottomed hollowportion T upwardly oriented during the stop of the engine is capable ofholding and retaining the lubricant oil which is splashed in thetransmission case M in accordance with the operation of the differentialdevice D and the like before the stop, and the lubricant oil which dripsfrom the ceiling wall Mt of the transmission case M after attaching tothe ceiling wall Mt in accordance with the operation of the differentialdevice D and the like before the stop. In addition, when thedifferential device D starts its operation, the lubricant oil held inthe bottomed hollow portion T can be quickly supplied to the rotationsliding portion rs between the pinion P and the pinion shaft PS via theoil guiding holes G due to the centrifugal force. In this case, sincethe oil guiding holes G extend from the inner periphery to the outerperiphery of the peripheral wall of the bottomed hollow portion T whileinclined outward in the axial direction of the pinion shaft PS, thelubricant oil held and retained in the bottomed hollow portion T can beeffectively inhibited from flowing out while the differential device Dstops, and can be efficiently supplied to the rotation sliding portionrs via the oil guiding holes G using the centrifugal force when thedifferential device D starts its operation.

It should be noted that depending on where the differential device Dstops, there is likelihood that: the bottomed hollow portion T isoriented horizontally; and it is accordingly difficult for the lubricantoil to be held in the bottomed hollow portion T. In most cases, however,either of the plurality of bottomed hollow portions T is directed upwardby being oriented vertically or inclinedly, and accordingly can hold thelubricant oil having splashed in the transmission case M and thelubricant oil having dripped from the ceiling wall Mt of thetransmission case M.

Furthermore, in the embodiment, the plurality of first oil intake holesand the plurality of second oil intake holes H2 are formed in the outerperipheral wall, that is, the case main body 4, of the differential caseDC so as to each have a circular shape in the cross section and bearranged at intervals in a peripheral direction of the differential caseDC, the first oil intake holes H1 and the second oil intake holes H2passing through the case main body 4 in the inside-outside direction andbeing capable of taking the lubricant oil in the transmission case M,for example the lubricant oil dripping from the ceiling wall Mt of thetransmission case M, into the differential case DC. Moreover, as clearlyshown in FIG. 2, the first and second oil intake holes H1, H2 are placedat their respective positions offset from intermediate points m betweenthe two adjacent pinions P in the peripheral direction of thedifferential case DC toward the pinions P.

Besides, the oil intake holes H1, H2 are formed such that as seen in aprojection plane orthogonal to the rotation axis L of the differentialcase DC, axes of the oil intake holes H1, H2 from inner opening ends Hito outer opening ends Ho of the oil intake holes H1, H2 are inclinedforward in the rotational direction R of the differential case DC whilethe vehicle is travelling forward. In addition, as seen in theprojection plane, the pinions P are placed outside areas A interposedbetween first imaginary lines L1 and second imaginary lines L2. In thisrespect, the first imaginary lines L1 join the rotation axis L and oneends in the peripheral direction of the inner opening ends Hi of the oilintake holes H1, H2, while the second imaginary lines L2 join therotation axis L and the other ends in the peripheral direction of theinner opening ends Hi of the oil intake holes H1, H2.

In addition, the embodiment employs the thin differential structure inwhich as described above, the diameter of the pinions P can be madesufficiently smaller than the diameter of the side gears S. For thisreason, even if the oil intake holes H1, H2 are arranged to be offsetfrom the intermediate points m toward the pinions P (that is, closer tothe pinions P) in the peripheral direction of the differential case DC,the pinions P can be arranged outside the areas A corresponding to theinner opening ends Hi of the oil intake holes H1, H2 withoutdifficulties. In other words, the pinions P are formed with a diametersufficiently smaller than the diameter of the side gears S such that thepinions P can be arranged outside the areas A without difficulties evenif the oil intake holes H1, H2 are arranged to be offset closer to thepinions P.

Because of this specialized oil intake holes H1, H2 in the outerperipheral wall of the differential case DC, while the vehicle istravelling forward and the differential case DC is rotating in thenormal rotation direction R at relatively low speed, the lubricant oildripping from the ceiling wall Mt of the transmission case M can beefficiently taken into the differential case DC via the plurality offirst oil intake holes H1 and the plurality of second oil intake holesH2 all inclined in their specific directions (that is, the directionswhich enable the lubricant oil to be efficiently taken into thedifferential case DC). Furthermore, among the oil intake holes H1, H2,particularly the first oil intake holes H1 which are arranged at a frontside in the normal rotation direction R of the pinions P and offset fromthe intermediate points m toward the pinions P are capable ofefficiently supplying the lubricant oil, which is dripped from theceiling wall Mt and is taken into the differential case DC, to themeshing portions of the pinions P and the side gears S near the firstoil intake holes H1. On the other hand, the second oil intake holes H2which are arranged at a back side in the normal rotation direction R ofthe pinions P and offset from the intermediate points m toward thepinions P are capable of supplying the lubricant oil, which is drippedfrom the ceiling wall Mt and is taken into the differential case DC, toan outer peripheral portion of the pinion shaft PS near the rotationcenter L of the differential case DC without the pinions P hindering thesupply of the lubricant oil (that is, without the pinions P working asobstacles which block the lubricant oil passages). From the outerperipheral portion thereof, the lubricant oil flows along an outerperipheral surface of the pinion shaft PS toward the outer ends of thepinion shaft PS, that is, toward the rotation sliding portions rsbetween the pinions P and the pinion shaft PS due to the centrifugalforce. Thereby, the lubricant oil can be efficiently supplied also tothe rotation sliding portions rs. As a result of these, the lubricantoil dripped from the ceiling wall Mt of the transmission case M isefficiently supplied to not only the meshing portions of the pinions Pwhich are in mesh with the side gears S, but also the rotation slidingportions rs between the pinions P and the pinion shaft PS. Thereby, theoverall lubricating effect can be enhanced. Incidentally, part of thelubricant oil which is dripped from the ceiling wall Mt and is takeninto the differential case DC via the oil intake holes H1, H2 reachesalso the inner side surfaces of the intermediate wall portions Sw of theside gears S, and flows due to the centrifugal force in the radiallyoutward direction, that is, toward the tooth portions Sg, along theinner side surfaces of the intermediate wall portions Sw.

Meanwhile, as described above, the washers W are installed between theinner side surfaces of the side wall portions Cs of the cover portionsC, C′ in the differential case DC and outer side surfaces of the sidegears S. For the purpose of positioning and retaining the washers W inappropriate fixed positions in consideration of the lubricant oilpassages to the penetrating oil passages 15, washer retaining grooves 16each having an annular shape are formed in at least one of the innerside surfaces of the side wall portions Cs and the outer side surfacesof the side gears S which mutually face each other (in the illustratedexample, the outer side surfaces of the side gears S). The washers W arefitted in the washer retaining grooves 16. In addition, relativepositions between the washers W and the penetrating oil passages 15 areset such that inner peripheral portions of the washers W face openingportions of the penetrating oil passages 15 in the outer side surfacesof the intermediate wall portions Sw. Thereby, the washers W inhibit theflow of the lubricant oil which tends to flow in the radially outwarddirection due to the centrifugal force in a gap between the inner sidesurfaces of the side wall portions Cs of the cover portions C, C′ andthe outer side surfaces of the side gears S. Thus, the lubricant oil canbe guided from inner peripheries of the washers W to the insides of theside gears S via the penetrating oil passages 15. For this reason, it ispossible to increase the amount of lubricant oil which passes throughthe penetrating oil passages 15, subsequently flows in the radiallyoutward direction along the inner side surfaces of the side gears S andeventually reaches the tooth portions Sg.

Moreover, referring to FIGS. 4 and 5 together, oil guide grooves 17 areprovided in a recess shape in the inner side surfaces of the side wallportions Cs of the cover portions C, C′, the oil guide grooves 17 beingcapable of guiding the flow of the lubricant oil into the washers W andthe penetrating oil passages 15 from peripheral edges of lighteningportions 8 while the differential case DC is in rotation. Each oil guidegroove 17 is formed in a substantially triangle shape by including: afirst inner side wall 17 a obliquely extending with respect to a tangentdirection of the corresponding oil retaining portion 7 (more concretely,obliquely extending to the center axis L side as going backward in thenormal rotation direction to be described later of the differential caseDC) from the peripheral edge of the corresponding lightening portion 8;a second inner side wall 17 b extending from the peripheral edge of thelightening portion 8 in the tangent direction of the oil retainingportion 7; and a back wall portion 17 c connecting inner ends of theboth inner side walls 17 a, 17 b. Furthermore, as seen in the projectionplane orthogonal to the rotation axis L of the differential case DC, aninner back groove portion 17 i of the oil guide groove 17 which the backwall portion 17 c faces is placed in a position which enables the innerback groove portion 17 i to always overlap part of the washer W, and totemporarily overlap the opening portions of the penetrating oil passages15 in the outer side surface of the intermediate wall portion Sw inaccordance with rotation of the differential case DC.

Thus, while the differential case DC is being rotated in the normalrotation direction R by the rotational force which is transmitted fromthe engine via the speed reduction gear mechanism RG in order to makethe automobile travel forward, the lubricant oil splashed around thedifferential case DC inside the transmission case M flows from theperipheral edges of the lightening portions 8 into the oil retainingportions 7 (that is, the oil guide grooves 17) due to relative speeddifference between the lubricant oil and the rotating cover portions C,C′. In this case, the lubricant oil flowing in the oil retainingportions 7 is efficiently collected toward the inner back grooveportions 17 i which are located in the rearmost positions in therotational direction in the oil guide grooves 17, particularly due to aguiding effect of the first inner side walls 17 a, and is efficientlyguided from the inner back groove portions 17 i to the washers W and thepenetrating oil passages 15. Subsequently, the lubricant oil passesthrough the penetrating oil passages 15 and reaches the inner sidesurfaces of the intermediate wall portions Sw of the side gears S.Thereafter, the lubricant oil flows in the radially outward directionalong the inner side surfaces of the intermediate wall portions Sw dueto the centrifugal force, as described above. Oil passages from thelightening portions 8 to the penetrating oil passages 15 via the oilguide grooves 17 form oil introduction passages Os for introducing thelubricant oil to the inner side surfaces of the intermediate wallportions Sw of the side gears S (that is, the mutually-facing surfacesof the pair of side gears S).

Besides, the cover portions C, C′ of the embodiment have an oil guidinginclined surface f in a peripheral edge portion of each lighteningportion 8, the oil guiding inclined surface f being capable of guidingflow of the lubricant oil into an inner side of the case main body 4during the rotation of the differential case DC. In addition, an inletof each oil guide groove 17 is opened to the oil guiding inclinedsurface f. As seen in a cross-section crossing the oil retainingportions 7 and the connecting arm portions 9 in the peripheral directionof the differential case DC (see the partial sectional views in FIGS. 4and 5), the oil guiding inclined surface f is formed so as to beinclined to the respective center sides in the peripheral direction ofthe oil retaining portion 7 and the connecting arm portion 9, towardtheir respective inner side surfaces from their respective outer sidesurfaces. Thus, by oil induction operation of the oil guiding inclinedsurface f, it is possible for the lubricant oil to smoothly flow fromthe outer side to the inner side of each of the cover portions C, C′ inaccordance with rotation of the differential case DC, and particularlyto effectively flow into the oil guide groove 17 from the inlet openedto the oil guiding inclined surface f.

Next, descriptions will be provided for an operation of the embodimentdescribed above. In the differential device D of the embodiment, in acase where the differential case DC receives rotational force from apower source (for example, an engine) via a speed reduction gearmechanism RG, when the pinion P revolves around the rotation axis L ofthe differential case DC together with the differential case DC, withoutrotating around the pinion shaft PS, the left and right side gears S arerotationally driven at the same speed, and their driving forces areevenly transmitted to the left and right output shafts J. Meanwhile,when a difference in rotational speed occurs between the left and rightoutput shafts J due to turn traveling or the like of the automobile, thepinion P revolves around the rotation axis L of the differential case DCwhile rotating around the pinion shaft PS. Thereby, the rotationaldriving force is transmitted from the pinion P to the left and rightside gears S while allowing differential rotations. The above is thesame as the operation of the conventional differential device.

Meanwhile, in a case where the power of the engine is being transmittedto the left and right output shafts J via the speed reduction gearmechanism RG and the differential device D while the automobile istravelling forward, the lubricant oil is powerfully splashed in variousareas inside the transmission case M due to the rotation of the movableelements of the speed reduction gear mechanism RG and the rotation ofthe differential case DC. As described above, part of the splashedlubricant oil flows into inner sides of the cover portions C, C′ via thelightening portions 8.

In this case, as described above, the lubricant oil flowing into the oilguide grooves 17 formed in the inner side surfaces of the side wallportions Cs of the cover portions C, C′ is efficiently collected towardthe inner back groove portions 17 i due to the guiding effect of thefirst inner side walls 17 a, and is efficiently guided from the innerback groove portions 17 i to the washers W and the penetrating oilpassages 15. For this reason, not only can the effect of lubricating thewashers W be enhanced, but also a sufficiently large amount of lubricantoil passing through the penetrating oil passages 15 and reaching theinner side surfaces of the intermediate wall portions Sw of the sidegears S can be secured. After reaching there, the lubricant oil flows inthe radially outward direction along the inner side surfaces of theintermediate wall portions Sw due to the centrifugal force, as describedabove. Part of the lubricant oil flow is splashed from the step portionsE having the edge shape to the spaces 60, and is caught and held in theoil reserving portions 61. Thereby, the rotation sliding portions rsbetween the pinion shaft PS and the pinions P are lubricated. On theother hand, the remaining part of the lubricant oil flow flows along thestep surfaces of the step portions E, and reaches the tooth portions Sgof the side gears S. Thereby, the meshing portions of the tooth portionsSg and the pinions P can be lubricated. As a result, even in a casewhere the tooth portions Sg of the side gears S place farther from theoutput shafts J due to increase in the diameter of the side gears S, oreven under severe driving conditions such as the high-speed rotation ofthe pinions P, the lubricant oil can be efficiently supplied to themeshing portions and the rotation sliding portions rs. Accordingly, theseizure in the meshing portions and the rotation sliding portions rs canbe prevented effectively.

Moreover, in the embodiment, the bottomed hollow portions T opened tothe inner space 1 of the transmission case M and capable of functioningas the oil reservoir are provided in a recess shape on the outer endsurfaces PSf of the pinion shaft PS. For this reason, while the enginestops, an upward-oriented one of the bottomed hollow portions T can holdand retain the lubricant oil which is splashed in the transmission caseM in accordance with the operation of the differential device D and thelike before the engine stops, and the lubricant oil which is drippedfrom the ceiling wall Mt of the transmission case M after being attachedto the ceiling wall Mt in accordance with the operation of thedifferential device D and the like before the engine stops. Accordingly,when the differential device D starts its operation, the lubricant oilheld in the bottomed hollow portion T can be quickly supplied to therotation sliding portions rs between the pinions P and the pinion shaftPS via the oil guiding holes G in the peripheral walls of the bottomedhollow portions T due to the centrifugal force. Thus, from the beginningof the start of the operation, the rotation sliding portions rs betweenthe pinions P and the pinion shaft PS can be sufficiently lubricatedwithout delay.

In addition, in the embodiment, the plurality of first oil intake holesand the plurality of second oil intake holes H2 each capable of takingthe lubricant oil dripped from the ceiling wall Mt of the transmissioncase M into the differential case DC are formed in the outer peripheralwall of the differential case DC; and the positions and directions inwhich the first and second oil intake holes H1, H2 are formed are asdescribed above. For these reasons, while the vehicle is travellingforward and the differential case DC is rotating in the normal rotationdirection R at relatively low speed, the lubricant oil dripped from theceiling wall Mt of the transmission case M can be efficiently taken intothe differential case DC via the first and second oil intake holes H1,H2. Furthermore, the first oil intake holes H1, which are arranged atthe front side in the normal rotation direction R of the pinions P andoffset from the intermediate points m between the mutually-adjacentpinions P toward the pinions P, are capable of efficiently supplying thelubricant oil, which is taken into the differential case DC, to themeshing portions of the pinions P and the side gears S near the firstoil intake holes H1. On the other hand, the second oil intake holes H2,which are arranged at the back side in the normal rotation direction Rof the pinions P and offset from the intermediate points m toward thepinions P, are capable of supplying the lubricant oil, which is drippedfrom the ceiling wall Mt and is taken into the differential case DC, tothe outer peripheral portion of the pinion shaft PS near the rotationcenter L of the differential case DC without the pinions P hindering thesupply of the lubricant oil. From the outer peripheral portion thereof,the lubricant oil flows along the outer peripheral surface of the pinionshaft PS toward the outer ends of the pinion shaft PS due to thecentrifugal force. Thereby, the lubricant oil can be efficientlysupplied also to the rotation sliding portions rs between the pinionshaft PS and the pinions P. As a result of these, the lubricant oildripped from the ceiling wall Mt of the transmission case M isefficiently supplied to not only the meshing portions of the pinions Pwhich are in mesh with the side gears S, but also the rotation slidingportions rs between the pinions P and the pinion shaft PS. Thereby, theoverall lubricating effect can be enhanced more.

Besides, part of an outer peripheral portion of the differential case DCof the embodiment may be, or does not have to be, immersed under the oilsurface of the lubricant oil held in an inner bottom portion of thetransmission case M. In the case where the part of the outer peripheralportion of the differential case DC is immersed under the oil surfacethereof, when the vehicle is traveling forward and the differential caseDC is rotating in the normal rotation direction R, the lubricant oiltaken into the differential case DC via the oil intake holes H1, H2 andheld in the differential case DC can be efficiently scooped up. For thisreason, the parts inside the differential case DC can be lubricated moreefficiently.

Meanwhile, in the conventional differential devices (particularly, theconventional differential devices each including a pinion (differentialgear) inside a differential case, and a pair of side gears (outputgears) meshing with the pinion (differential gear)) exemplified inJapanese Patent No. 4803871 and Japanese Patent Application KOKAIPublication No. 2002-364728, the number Z1 of teeth of the side gear(output gear) and the number Z2 of teeth of the pinion (differentialgear) are generally set at 14 and 10, 16 and 10, or 13 and 9,respectively, as shown in Japanese Patent Application KOKAI PublicationNo. 2002-364728, for example. In these cases, the number-of-teeth ratiosZ1/Z2 of the output gears to the differential gears are 1.4, 1.6 and1.44, respectively. In addition, other publicly-known examples of thecombination of the number Z1 of teeth and the number Z2 of teeth forconventional differential devices include 15 and 10, 17 and 10, 18 and10, 19 and 10, and 20 and 10. In these cases, the number-of-teeth ratiosZ1/Z2 are at 1.5, 1.7, 1.8, 1.9 and 2.0, respectively.

On the other hand, nowadays, there is an increase in the number oftransmission systems which are under layout restrictions around theirrespective differential devices. Accordingly, the market demands thatdifferential devices be sufficiently reduced in width (i.e., thinned) inthe axial direction of their output shafts while securing the gearstrength for the differential devices. However, the structural forms ofthe conventional existing differential devices are wide in the axialdirection of the output shafts, as apparent from the gear combinationsleading to the above-mentioned number-of-teeth ratios. This makes itdifficult to satisfy the market demand.

With this taken into consideration, an attempt to find a concreteconfiguration example of the differential device D which can besufficiently reduced in width (i.e., thinned) in the axial direction ofthe output shafts while securing the gear strength for the differentialdevice has been made as follows, from a viewpoint different from that ofthe foregoing embodiment. Incidentally, the structures of the componentsof the differential device D of this configuration example are the sameas the structures of the components of the differential device D of theforegoing embodiment which has been described using FIGS. 1 to 6. Forthis reason, the components of the configuration example will be denotedwith the same reference signs as those of the embodiment, anddescriptions for the structures will be omitted.

To begin with, let us explain a basic concept for sufficiently reducingthe width of (i.e., thinning) the differential device D in the axialdirection of the output shafts J referring to FIG. 7 together. Theconcept is as follows.

Approach [1] To make the number-of-teeth ratio Z1/Z2 of the side gear S,that is, the output gear to the pinion P, that is, the differential gearlarger than the number-of-teeth ratio used for the conventional existingdifferential device. (This leads to a decrease in the module(accordingly the tooth thickness) of the gear and a resultant decreasein the gear strength, while leading to an increase in the pitch circlediameter of the side gear S, a resultant decrease in transmission loadin the meshing portion of the gear, and a resultant increase in the gearstrength. However, the gear strength as a whole decreases, as discussedbelow.)Approach [2] To make the pitch cone distance PCD of the pinion P largerthan the pitch cone distance in the conventional existing differentialdevice. (This leads to an increase in the module of the gear and aresultant increase in the gear strength, while leading to an increase inthe pitch circle diameter of the side gear S, a resultant decrease inthe transmission load in the meshing portion of the gear, and aresultant increase in the gear strength. Thus, the gear strength as awhole increases greatly, as discussed below.)

For these reasons, when the number-of-teeth ratio Z1/Z2 and the pitchcone distance PCD are set such that the amount of decrease in the gearstrength based on Approach [1] is equal to the amount of increase in thegear strength based on Approach [2] or such that the amount of increasein the gear strength based on Approach [2] is greater than the amount ofdecrease in the gear strength based on Approach [1], the gear strengthas a whole can be made equal to or greater than that of the conventionalexisting differential device.

Next, let us concretely examine how the gear strength changes based onApproaches [1] and [2] using mathematical expressions. Incidentally, theexamination will be described in the following embodiment. First of all,a “reference differential device” is defined as a differential device D′in which the number Z1 of teeth of the side gear S is set at 14 whilethe number Z2 of teeth of the pinion P is set at 10. In addition, foreach variable, a “change rate” is defined as a rate of change in thevariable in comparison with the corresponding base number (i.e., 100%)of the reference differential device D′.

Approach [1]

When MO, PD₁, θ₁, PCD, F, and TO respectively denote the module, pitchcircle diameter, pitch angle, pitch cone distance, transmission load inthe gear meshing portion, and transmission torque in the gear meshingportion, of the side gear S, general formulae concerning the bevel gearprovideMO=PD₁ /Z1,PD₁=2PCD·sin θ₁, andθ₁=tan⁻¹(Z1/Z2).

From these expressions, the module of the gear is expressed withMO=2PCD·sin { tan⁻¹(Z1/Z2)}/Z1  (1)

Meanwhile, the module of the reference differential device D′ isexpressed with2PCD·sin { tan⁻¹(7/5)}/14.

Dividing the term on the right side of Expression (1) by 2PCD·sin {tan⁻¹(7/5)}/14 yields a module change rate with respect to the referencedifferential device D′, which is expressed with Expression (2) givenbelow.

$\begin{matrix}{{{Module}\mspace{14mu}{Change}\mspace{14mu}{Rate}} = \frac{14 \cdot {\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}{Z\;{1 \cdot {\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}}}} & (2)\end{matrix}$

In addition, the section modulus of the tooth portion corresponding tothe gear strength (i.e., the bending strength of the tooth portion) isin proportion to the square of the tooth thickness, while the tooththickness has a substantially linear relationship with the module MO.For these reasons, the square of the module change rate corresponds to arate of change in the section modulus of the tooth portion, accordinglya gear strength change rate. In other words, based on Expression (2)given above, the gear strength change rate is expressed with Expression(3) given below. Expression (3) is represented by a line L1 in FIG. 8when the number Z2 of teeth of the pinion P is 10. From the line L1, itis learned that as the number-of-teeth ratio Z1/Z2 becomes larger, themodule becomes smaller and the gear strength accordingly becomes lower.

$\begin{matrix}\begin{matrix}{{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}\mspace{14mu}{Rate}} = \left( {{Module}\mspace{14mu}{Change}\mspace{14mu}{Rate}} \right)^{2}} \\{= \frac{196 \cdot {\sin^{2}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}{Z\;{1^{2} \cdot {\sin^{2}\left( {\tan^{- 1}\frac{7}{5}} \right)}}}}\end{matrix} & (3)\end{matrix}$

Meanwhile, based on the general formulae concerning the bevel gear, atorque transmission distance of the side gear S is expressed withExpression (4) given below.PD₁/2=PCD·sin { tan⁻¹(Z1/Z2)}  (4)

From the torque transmission distance PD₁/2, the transmission load F isgiven asF=2TO/PD₁.For this reason, when the torque TO of the side gear S of the referencedifferential device D′ is constant, the transmission load F is ininverse proportion to the pitch circle diameter PD₁. In addition, therate of change in the transmission load F is in inverse proportion tothe gear strength change rate. For this reason, the gear strength changerate is equal to the rate of change in the pitch circle diameter PD₁.

As a result, using Expression (4), the rate of change in the pitchcircle diameter PD₁ is expressed with Expression (5) given below.

$\begin{matrix}\begin{matrix}{{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}\mspace{14mu}{Rate}} = {{PD}_{1}\mspace{14mu}{Change}\mspace{14mu}{Rate}}} \\{= \frac{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}}\end{matrix} & (5)\end{matrix}$

Expression (5) is represented by a line L2 in FIG. 8 when the number Z2of teeth of the pinion P is 10. From the line L2, it is learned that asthe number-of-teeth ratio Z1/Z2 becomes larger, the transmission loadbecomes smaller, and the gear strength accordingly becomes stronger.

Eventually, the gear strength change rate in accordance with theincrease in the number-of-teeth ratio Z1/Z2 is expressed with Expression(6) given below by multiplying a rate of decrease change in the gearstrength in accordance with the decrease in the module MO (the term onthe right side of Expression (3) shown above) and a rate of increasechange in the gear strength in accordance with the decrease in thetransmission load (the term on the right side of Expression (5) shownabove).

$\begin{matrix}{\begin{matrix}{{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}}\mspace{31mu}} \\{{Rate}\mspace{14mu}{in}\mspace{14mu}{Accordance}\mspace{14mu}{with}} \\{{{Number}\text{-}{of}\text{-}{Teeth}\mspace{14mu}{Ratio}}\mspace{14mu}}\end{matrix} = \frac{196 \cdot {\sin^{3}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}{Z\;{1^{2} \cdot {\sin^{3}\left( {\tan^{- 1}\frac{7}{5}} \right)}}}} & (6)\end{matrix}$

Expression (6) is represented by a line L3 in FIG. 8 when the number Z2of teeth of the pinion P is 10. From the line L3, it is learned that asthe number-of-teeth ratio Z1/Z2 becomes larger, the gear strength as awhole becomes lower.

Approach [2]

In a case of increasing the pitch cone distance PCD of the pinion P morethan the pitch cone distance in the reference differential device D′,when PCD1, PCD2 respectively denote the pitch cone distance PCD beforethe change and the pitch cone distance PCD after the change, the modulechange rate in accordance with the change in the pitch cone distance PCDis expressed withPCD2/PCD1if the number of teeth is constant, based on the above-mentioned generalformulae concerning the bevel gear.

Meanwhile, as being clear from the above-discussed process for derivingExpression (3), the gear strength change rate of the side gear Scorresponds to the square of the module change rate. For this reason,Gear Strength Change Rage in Accordance with Increase inModule=(PCD2/PCD1)²  (7)is obtained. Expression (7) is represented by a line L4 in FIG. 9. Fromthe line L4, it is learned that as the pitch cone distance PCD becomeslarger, the module becomes larger, and the gear strength accordinglybecomes stronger.

In addition, when the pitch cone distance PCD is made larger than thepitch cone distance PCD1 in the reference differential device D′, thetransmission load F decreases. Thereby, the gear strength change ratebecomes equal to the rate of change in the pitch circle diameter PD₁, asdescribed above. In addition, the pitch circle diameter PD₁ of the sidegear S is in proportion to the pitch cone distance PCD. For thesereasons,Gear Strength Change Rate in Accordance with Decrease in TransmissionLoad=PCD2/PCD1  (8)is obtained.

Expression (8) is represented by a line L5 in FIG. 9. From the line L5,it is learned that as the pitch cone distance PCD becomes larger, thetransmission load becomes lower, and the gear strength accordinglybecomes stronger.

In addition, the gear strength change rate in accordance with theincrease in the pitch cone distance PCD is expressed with Expression (9)given below by multiplying the rate of increase change in the gearstrength in accordance with the increase in the module MO (the term onthe right side of Expression (7) shown above) and the rate of increasechange in the gear strength in accordance with the decrease in thetransmission load in response to the increase in the pitch circlediameter PD (the term on the right side of Expression (8) shown above).Gear Strength Change Rate in Accordance with Increase in Pitch ConeDistance=(PCD2/PCD1)³  (9)

Expression (9) is represented by a line L6 in FIG. 9. From the line L6,it is learned that as the pitch cone distance PCD becomes larger, thegear strength is increased greatly.

With these taken into consideration, the combination of thenumber-of-teeth ratio Z1/Z2 and the pitch cone distance PCD isdetermined such that: the decrease in the gear strength based onApproach [1] given above (the increase in the number-of-teeth ratio) issufficiently compensated for by the increase in the gear strength basedon Approach [2] given above (the increase in the pitch cone distance) soas to make the overall gear strength of the differential device equal toor greater than the gear strength of the conventional existingdifferential device.

For example, 100% of the gear strength of the side gear S of thereference differential device D′ can be kept by setting the gearstrength change rate in accordance with the increase in thenumber-of-teeth ratio (i.e., the term on the right side of Expression(6) given above) obtained based on Approach [1] given above and the gearstrength change rate in accordance with the increase in the pitch conedistance (i.e., the term on the right side of Expression (9) givenabove) obtained based on Approach [2] given above, such that themultiplication of these gear strength change rates becomes equal to100%. Thereby, the relationship between the number-of-teeth ratio Z1/Z2and the rate of change in the pitch cone distance PCD for keeping 100%of the gear strength of the reference differential device D′ can beobtained from Expression (10) given below. Expression (10) isrepresented by a line L7 in FIG. 10 when the number Z2 of teeth of thepinion P is 10.

$\begin{matrix}\begin{matrix}{{{PCD}\; 2\text{/}{PCD}\; 1} = \left( {100\%\text{/}\begin{matrix}{{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}}\mspace{31mu}} \\{{Rate}\mspace{14mu}{in}\mspace{14mu}{Accordance}\mspace{14mu}{with}} \\{{{Number}\text{-}{of}\text{-}{Teeth}\mspace{14mu}{Ratio}}\mspace{14mu}}\end{matrix}} \right)^{\frac{1}{3}}} \\{= \left\{ \frac{\frac{1}{196 \cdot {\sin^{3}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}{Z\;{1^{2} \cdot {\sin^{3}\left( {\tan^{- 1}\frac{7}{5}} \right)}}} \right\}^{\frac{1}{3}}} \\{= {\left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}\end{matrix} & (10)\end{matrix}$

Like this, Expression (10) represents the relationship between thenumber-of-teeth ratio Z1/Z2 and the rate of change in the pitch conedistance PCD for keeping 100% of the gear strength of the referencedifferential device D′ when the number-of-teeth ratio Z1/Z2 is equal to14/10 (see FIG. 10). The rate of change in the pitch cone distance PCDrepresented by the vertical axis in FIG. 10 can be converted into aratio of d2/PCD where d2 denotes a shaft diameter of the pinion shaft PS(i.e., the pinion support portion) supporting the pinion P.

TABLE 1 PCD SHAFT DIAMETER (d2) d2/PCD 31 13 42% 35 15 43% 38 17 45% 3917 44% 41 18 44% 45 18 40%

To put it concretely, in the conventional existing differential device,the increase change in the pitch cone distance PCD correlates with theincrease change in the shaft diameter d2 as shown in Table 1, and can berepresented by a decrease in the ratio of d2/PCD when d2 is constant. Inaddition, in the conventional existing differential device, d2/PCD fallswithin a range of 40% to 45% as shown in Table 1 given above when theconventional existing differential device is the reference differentialdevice D′, and the gear strength increases as the pitch cone distancePCD increases. Judging from these, the gear strength of the differentialdevice can be made equal to or greater than the gear strength of theconventional existing differential device by determining the shaftdiameter d2 of the pinion shaft PS and the pitch cone distance PCD suchthat at least d2/PCD is equal to or less than 45%, when the differentialdevice is the reference differential device D′. In other words, when thedifferential device is the reference differential device D′, it sufficesif d2/PCD≦0.45 is satisfied. In this case, when PCD2 denotes the pitchcone distance PCD which is changed to become larger or less than thepitch cone distance PCD1 of the reference differential device D′, itsuffices ifd2/PCD2≦0.45/(PCD2/PCD1)  (11)is satisfied. Furthermore, the application of Expression (11) toExpression (10) given above can convert the relationship between d2/PCDand the number-of-teeth ratio Z1/Z2 into Expression (12) given below.

$\begin{matrix}\begin{matrix}{{d\; 2\text{/}{PCD}} \leqq {0.45\text{/}\left( {{PCD}\; 2\text{/}{PCD}\; 1} \right)}} \\{= {0.45\text{/}\left\{ {\left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}} \right\}}} \\{= {0.45 \cdot \left( \frac{14}{Z\; 1} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}}}\end{matrix} & (12)\end{matrix}$

When the Expression (12) is equal, Expression (12) can be represented bya line L8 in FIG. 11 if the number Z2 of teeth of the pinion P is 10.When the Expression (12) is equal, the relationship between d2/PCD andthe number-of-teeth ratio Z1/Z2 keeps 100% of the gear strength of thereference differential device D′.

Meanwhile, in conventional existing differential devices, usually, notonly the number-of-teeth ratio Z1/Z2 equal to 1.4 used above to explainthe reference differential device D′ but also the number-of-teeth ratioZ1/Z2 equal to 1.6 or 1.44 is adopted. This needs to be taken intoconsideration. Based on the assumption that the reference differentialdevice D′ (Z1/Z2=1.4) guarantee the necessary and sufficient gearstrength, that is, 100% of gear strength, it is learned, as being clearfrom FIG. 8, that the gear strength of conventional existingdifferential devices in which the number-of-teeth ratio Z1/Z2 is 16/10is as low as 87% of the gear strength of the reference differentialdevice D′. The general practice, however, is that the gear strength lowat that level is accepted as practical strength and actually used forconventional existing differential devices. Judging from this, one mayconsider that gear strength which needs to be sufficiently secured forand is acceptable for the differential device which is thinned in theaxial direction is at least equal to, or greater than, 87% of the gearstrength of the reference differential device D′.

From the above viewpoint, first, a relationship for keeping 87% of thegear strength of the reference differential device D′ is obtainedbetween the number-of-teeth ratio Z1/Z2 and the rate of change in thepitch cone distance PCD. The relationship can be expressed withExpression (10′) given below by performing a calculation by emulatingthe process of deriving Expression (10) given above (i.e., a calculationsuch that the multiplication of the gear strength change rate inaccordance with the increase in the number-of-teeth ratio (i.e., theterm on the right side of Expression (6) given above) and the gearstrength change rate in accordance with the increase in the pitch conedistance (i.e., the term on the right side of Expression (9) givenabove) becomes equal to 87%).

$\begin{matrix}\begin{matrix}{{{PCD}\; 2\text{/}{PCD}\; 1} = \left( {87\%\text{/}\begin{matrix}{{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}}\mspace{31mu}} \\{{Rate}\mspace{14mu}{in}\mspace{14mu}{Accordance}\mspace{14mu}{with}} \\{{{Number}\text{-}{of}\text{-}{Teeth}\mspace{14mu}{Ratio}}\mspace{14mu}}\end{matrix}} \right)^{\frac{1}{3}}} \\{= \left\{ \frac{\frac{0.87}{196 \cdot {\sin^{3}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}{Z\;{1^{2} \cdot {\sin^{3}\left( {\tan^{- 1}\frac{7}{5}} \right)}}} \right\}^{\frac{1}{3}}} \\{= {0.87^{\frac{1}{3}} \cdot \left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}\end{matrix} & \left( 10^{\prime} \right)\end{matrix}$

Thereafter, when Expression (11) given above is applied to Expression(10′) given above, the relationship between d2/PCD and thenumber-of-teeth ratio Z1/Z2 for keeping 87% or more of the gear strengthof the reference differential device D′ can be converted into Expression(13) given below. However, the calculation is performed using thefollowing rules that: the number of significant figures is three for allthe factors, except for factors expressed with variables; digits belowthe third significant figure are rounded down; and although the resultof the calculation cannot avoid approximation by an calculation error,the mathematical expression uses the equals sign because the error isnegligible.

$\begin{matrix}\begin{matrix}{{d\; 2\text{/}{PCD}} \leqq {0.45\text{/}\left\{ {0.87^{\frac{1}{3}} \cdot \left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}} \right\}}} \\{= {3.36 \cdot \left( \frac{1}{Z\; 1} \right)^{\frac{2}{3}} \cdot {\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}\end{matrix} & (13)\end{matrix}$

When the Expression (13) is equal, Expression (13) can be represented byFIG. 11 (more specifically, by a line L9 in FIG. 11) if the number Z2 ofteeth of the pinion P is 10. In this case, an area corresponding toExpression (13) is an area on and under the line L9 in FIG. 11. Inaddition, a specific area (a hatched area in FIG. 11) satisfyingExpression (13) and located on the right side of a line L10 in FIG. 11where the number-of-teeth ratio Z1/Z2>2.0 is satisfied is an area forsetting Z1/Z2 and d2/PCD which enable at least 87% or more of the gearstrength of the reference differential device D′ to be securedparticularly for the differential device thinned in the axial directionwhere the number Z2 of teeth of the pinion P is 10 and thenumber-of-teeth ratio Z1/Z2 is greater than 2.0. For reference, a blackdiamond in FIG. 11 represents an example where the number-of-teeth ratioZ1/Z2 and d2/PCD are set at 40/10 and 20.00%, respectively, and a blacktriangle in FIG. 11 represents an example where the number-of-teethratio Z1/Z2 and d2/PCD are set at 58/10 and 16.67%, respectively. Theseexamples fall within the specific area. A result of a simulation forstrength analysis on these examples has confirmed that the gear strengthequal to or greater than those of the conventional differential devices(more specifically, the gear strength equal to or greater than 87% ofthe gear strength of the reference differential device D′) wereobtained.

Thus, the thinned differential device falling within the specific areais configured as the differential device which, as a whole, issufficiently reduced in width in the axial direction of the outputshafts while securing the gear strength (for example, static torsionload strength) and the maximum amount of torque transmission atapproximately the same levels as the conventional existing differentialdevices which are not thinned in the axial direction thereof.Accordingly, it is possible to achieve effects of: being capable ofeasily incorporating the differential device in a transmission system,which is under many layout restrictions around the differential device,with great freedom and no specific difficulties; being extremelyadvantageous in reducing the size of the transmission system; and thelike.

Moreover, when the thinned differential device in the specific area has,for example, the structure of the above-mentioned embodiment (morespecifically, the structures shown in FIGS. 1 to 6), the thinneddifferential device in the specific area can obtain an effect derivedfrom the structure shown in the embodiment.

It should be noted that although the foregoing descriptions (thedescriptions in connection with FIGS. 8, 10, 11 in particular) have beenprovided for the differential device in which the number Z2 of teeth ofthe pinion P is set at 10, the present invention is not limited to this.For example, when the number Z2 of teeth of the pinion P is set at 6, 12and 20, too, the thinned differential device capable of achieving theabove effects can be represented by Expression (13), as shown by hatchedareas in FIGS. 12, 13 and 14. In other words, Expression (13) derived inthe above-described manner is applicable regardless of the change in thenumber Z2 of teeth of the pinion P. For example, even when the number Z2of teeth of the pinion P is set at 6, 12 and 20, the above effects canbe obtained by setting the number Z1 of teeth of the side gear S, thenumber Z2 of teeth of the pinion P, the shaft diameter d2 of the pinionshaft PS and the pitch cone distance PCD such that Expression (13) issatisfied, like in the case where the number Z2 of teeth of the pinion Pis set at 10.

Furthermore, for reference, a black diamond in FIG. 13 represents anexample where when the number Z2 of teeth of the pinion P is 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 48/12 and 20.00%,respectively, and a black triangle in FIG. 13 represents an examplewhere when the number Z2 of teeth of the pinion P is 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 70/12 and 16.67%,respectively. A result of a simulation for strength analysis on theseexamples has confirmed that the gear strength equal to or greater thanthose of the conventional differential devices (more specifically, thegear strength equal to or greater than 87% of the gear strength of thereference differential device D′) were obtained. Moreover, theseexamples fall within the specific area, as shown in FIG. 13.

As comparative examples, let us show examples which do not fall withinthe specific area. A white star in FIG. 11 represents an example wherewhen the number Z2 of teeth of the pinion P is for example 10, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 58/10 and 27.50%,respectively, and a white circle in FIG. 11 represents an example wherewhen the number Z2 of teeth of the pinion P is for example 10, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 40/10 and 34.29%,respectively. A white star in FIG. 13 represents an example where whenthe number Z2 of teeth of the pinion P is for example 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 70/12 and 27.50%,respectively, and a white circle in FIG. 13 represents an example wherewhen the number Z2 of teeth of the pinion P is for example 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 48/12 and 34.29%,respectively. A result of a simulation for strength analysis on theseexamples has confirmed that the gear strength equal to or greater thanthose of the conventional differential devices (more specifically, thegear strength equal to or greater than 87% of the gear strength of thereference differential device D′) were not obtained. In other words, theabove effects cannot be obtained from the examples which do not fallwithin the specific area.

Although the embodiment of the present invention have been described,the present invention is not limited to the foregoing embodiment.Various design changes may be made to the present invention within ascope not departing from the gist of the present invention.

For example, the foregoing embodiment has shown the differential devicein which: the speed reduction gear mechanism RG formed from theplanetary gear mechanism is adjacently placed on the one side of thedifferential case DC; the output-side element (carrier 53) of the speedreduction gear mechanism RG is connected to the differential case DC(cover portion C′); and the power from the power source is transmittedto the differential case DC via the speed reduction gear mechanism RG.However, an output-side element of a speed reduction gear mechanismformed from a gear mechanism other than the planetary gear mechanism maybe connected to the differential case DC.

Furthermore, without using the above mentioned speed reduction gearmechanism, an input tooth portion (final driven gear) receiving thepower from the power source may be integrally formed on, or afterwardfixed to, the outer peripheral portion of the differential case DC sothat the power from the power source is transmitted to the differentialcase DC via the input tooth portion. In this case, specific parts of theouter peripheral surface of the differential case DC, for example theopening portions of the hollow cylindrical portions T and the openingportions of the first and second oil intake holes H1, H2, are alwaysexposed to the inner space 1 of the transmission case M without beingcovered with the input tooth portion.

Moreover, the foregoing embodiment has shown the differential device inwhich the oil introduction passages Os guiding the lubricant oil to theinner side surfaces of the intermediate wall portions Sw of the sidegears S (that is, the mutually-facing surfaces of the pair of side gearsS) are, for example, the oil passages extending from the lighteningportions 8 formed in the side wall portions Cs of the cover portions C,C′ to the penetrating oil passages 15 via the oil guide grooves 17.Nevertheless, instead of such oil passages, or in addition to such oilpassages, other oil introduction passages are usable as well. Other oilintroduction passages Os may be obtained, for example, by: extending theboss portions Cb of the cover portions C, C′ of the differential case DCoutward beyond the shaft portions Sj of the side gears S in the axialdirection; rotatably fitting the output shafts J to inner peripheralsurfaces of the extension portions of the boss portions Cb; andproviding spiral grooves in a recess shape to at least one of thefitting surfaces of the extension portions of the boss portions Cb andthe output shafts J (for example, the inner peripheral surfaces of theextension portions of the boss portions Cb). Thereby, the lubricant oilexisting around the extension portions of the boss portions Cb in theinner space 1 of the transmission case M can be efficiently supplied tospline-fitting portions 6 between the shaft portions Sj of the sidegears S and the output shafts J, accordingly to the inner side surfacesof the intermediate wall portions Sw, via the spiral grooves while theboss portions Cb and the output shafts J are rotating relative to eachother. In this case, if lubricant oil passages extending in the axialdirection are formed by removing some spline teeth from thespline-fitting portions 6, the lubricant oil can be more efficientlysupplied to the inner side surfaces of the intermediate wall portions Swof the side gears S.

Furthermore, it should be noted that instead of the spiral grooves, orin addition to the spiral grooves, the lubricant oil may be pressure-fedand supplied from the oil pump to the spline-fitting portions 6 betweenthe shaft portions Sj of the side gears S and the output shafts J sothat the lubricant oil supplied under pressure is supplied to the innerside surfaces of the intermediate wall portions Sw of the side gears Svia the spline-fitting portions 6.

Moreover, the foregoing embodiment where the back surfaces of the pairof side gears S are covered with the pair of cover portions C, C′ hasbeen shown, however, in the present invention, the back surface of onlyone side gear S may be provided with the cover portion. In this case,for example, the drive member (for example, the carrier 53 of the speedreduction gear mechanism RG) situated upstream of a power transmissionpassage may be disposed on the side gear side provided with no coverportion so that the drive member and the differential case DC areconnected to each other.

In addition, although the foregoing embodiment has been shown in whichthe differential device D allows the difference in rotational speedbetween the left and right axles, the differential device of the presentinvention may be carried out as a center differential configured toabsorb the difference in rotational speed between front wheels and rearwheels.

Besides, the foregoing embodiment has described the differential devicein which the plurality of pinions P are supported by the single pinionshaft PS. Nevertheless, the present invention is also applicable to adifferential device in which the pinions are individually supported bymutually-separated pinion shafts, respectively.

What is claimed is:
 1. A differential device which distributively transmits rotational force of a differential case to a pair of output shafts, comprising: a pinion placed in the differential case; a pinion shaft supported in the differential case and penetrating through and rotatably supporting the pinion; a pair of side gears including at respective outer peripheral portions thereof gear portions which are placed in mesh with the pinion, the side gears facing each other with the pinion shaft interposed therebetween and being connected respectively to the pair of output shafts; and an oil introduction passage introducing lubricant oil to mutually-facing surfaces of the pair of side gears, wherein an oil reserving portion is formed in a portion where the pinion and the pinion shaft face each other such that the oil reserving portion communicates with a fitting portion between the pinion and the pinion shaft, where the pinion and the pinion shaft are rotationally slidable relative to each other, the oil reserving portion facing a space adjacent to an end surface of the pinion that is located on a radially inner side, the oil reserving portion being capable of catching and holding the lubricant oil splashed to the space, and a step portion is formed in at least one of the mutually-facing surfaces of the pair of side gears, the step portion separating part of the lubricant oil from a lubricant oil flow that flows due to centrifugal force in a radially outward direction along the one of the mutually-facing surfaces and guiding the part of the lubricant oil into the space.
 2. The differential device according to claim 1, wherein the side gears include: shaft portions connected respectively to the pair of output shafts; the gear portions separating outward from the shaft portions in the radial direction; and intermediate wall portions each having a flat shape and extending outward from inner end portions of the shaft portions in the radial direction.
 3. The differential device according to claim 2, wherein the step portion is formed such that an imaginary plane passes through any one of an inner space and an opening edge of the oil reserving portion, the imaginary plane passing through a top surface of the step portion and being orthogonal to a rotation axis of the differential case.
 4. The differential device according to claim 1, wherein the step portion is formed such that an imaginary plane passes through any one of an inner space and an opening edge of the oil reserving portion, the imaginary plane passing through a top surface of the step portion and being orthogonal to a rotation axis of the differential case.
 5. A differential device which distributively transmits rotational force of a differential case to a pair of output shafts, comprising: a pinion placed in the differential case; a pinion shaft supported in the differential case and penetrating through and rotatably supporting the pinion; a pair of side gears including at respective outer peripheral portions thereof gear portions which are placed in mesh with the pinion, the side gears facing each other with the pinion shaft interposed therebetween and being connected respectively to the pair of output shafts; and an oil introduction passage introducing lubricant oil to mutually-facing surfaces of the pair of side gears; a space interposed between recesses formed in the mutually-facing surfaces of the pair of side gears and an end surface of the pinion that is located on a radially inner side of the side gears; and an oil reserving portion formed in a portion where the pinion and the pinion shaft face each other such that the oil reserving portion faces the space, the oil reserving portion communicating with a fitting portion between the pinion and the pinion shaft, where the pinion and the pinion shaft are rotationally slidable relative to each other, wherein a step portion is formed in at least one of the mutually-facing surfaces of the pair of side gears, the step portion being located in an inner peripheral end of the space in the radial direction and forming an opening edge of the corresponding recess.
 6. The differential device according to claim 5, wherein the side gears include: shaft portions connected respectively to the pair of output shafts; the gear portions separating outward from the shaft portions in the radial direction; and intermediate wall portions each having a flat shape and extending outward from inner end portions of the shaft portions in the radial direction.
 7. The differential device according to claim 6, wherein the step portion is formed such that an imaginary plane passes through any one of an inner space and an opening edge of the oil reserving portion, the imaginary plane passing through a top surface of the step portion and being orthogonal to a rotation axis of the differential case.
 8. The differential device according to claim 2, wherein the step portion is formed such that an imaginary plane passes through any one of an inner space and an opening edge of the oil reserving portion, the imaginary plane passing through a top surface of the step portion and being orthogonal to a rotation axis of the differential case.
 9. A differential device which distributively transmits rotational force of a differential case to a pair of output shafts, comprising: a differential gear placed in the differential case; a differential gear support portion supported in the differential case and penetrating through and rotatably supporting the differential gear; a pair of output gears including at respective outer peripheral portions thereof gear portions which are placed in mesh with the differential gear, the output gears facing each other with the differential gear support portion interposed therebetween and being connected respectively to the pair of output shafts; and an oil introduction passage introducing lubricant oil to mutually-facing surfaces of the pair of output gears; a space interposed between recesses formed in the mutually-facing surfaces of the pair of output gears and an end surface of the differential gear that is located on a radially inner side of the output gears; and an oil reserving portion formed in a portion where the differential gear and the differential gear support portion face each other such that the oil reserving portion faces the space, the oil reserving portion communicating with a fitting portion between the differential gear and the differential gear support portion, where the differential gear and the differential gear support portion are rotationally slidable relative to each other, wherein a step portion is formed in at least one of the mutually-facing surfaces of the pair of output gears, the step portion being located in an inner peripheral end of the space in the radial direction and forming an opening edge of the corresponding recess, wherein ${d\; 2\text{/}{PCD}} \leqq {3.36 \cdot \left( \frac{1}{Z\; 1} \right)^{\frac{2}{3}} \cdot {\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}$ is satisfied, and Z1/Z2>2 is satisfied, where Z1, Z2, d2 and PCD denote the number of teeth of each of the output gears, the number of teeth of the differential gear, a diameter of the differential gear support portion and a pitch cone distance, respectively.
 10. The differential device according to claim 9, wherein Z1/Z2≧4 is satisfied.
 11. The differential device according to claim 10, wherein the step portion is formed such that an imaginary plane passes through any one of an inner space and an opening edge of the oil reserving portion, the imaginary plane passing through a top surface of the step portion and being orthogonal to a rotation axis of the differential case.
 12. The differential device according to claim 9, wherein Z1/Z2≧5.8 is satisfied.
 13. The differential device according to claim 12, wherein the step portion is formed such that an imaginary plane passes through any one of an inner space and an opening edge of the oil reserving portion, the imaginary plane passing through a top surface of the step portion and being orthogonal to a rotation axis of the differential case.
 14. The differential device according to claim 9, wherein the step portion is formed such that an imaginary plane passes through any one of an inner space and an opening edge of the oil reserving portion, the imaginary plane passing through a top surface of the step portion and being orthogonal to a rotation axis of the differential case. 